Combined physical and chemical etch to reduce magnetic tunnel junction (mtj) sidewall damage

ABSTRACT

A process flow for forming magnetic tunnel junction (MTJ) nanopillars with minimal sidewall residue and minimal sidewall damage is disclosed wherein a pattern is first formed in a hard mask that is an uppermost MTJ layer. Thereafter, the hard mask sidewall is etch transferred through the remaining MTJ layers including a reference layer, free layer, and tunnel barrier between the free layer and reference layer. The etch transfer may be completed in a single RIE step that features a physical component involving inert gas ions or plasma, and a chemical component comprised of ions or plasma generated from one or more of methanol, ethanol, ammonia, and CO. In other embodiments, a chemical treatment with one of the aforementioned chemicals, and a volatilization at 50° C. to 450° C. may follow an etch transfer through the MTJ stack with an ion beam etch or plasma etch involving inert gas ions.

PRIORITY DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 15/595,484, filed May 15, 2017, which is hereinincorporated by reference in its entirety.

RELATED PATENT APPLICATION

This application is related to U.S. Pat. No. 8,722,543; which isassigned to a common assignee and is herein incorporated by reference inits entirety.

TECHNICAL FIELD

The present disclosure relates to a method of reducing MTJ sidewalldamage during an etch process that transfers a mask pattern through aMTJ stack of layers thereby generating an array of MTJ nanopillars withimproved magnetoresistive ratio and other magnetic properties.

BACKGROUND

A MTJ memory element is also referred to as a MTJ nanopillar and is akey component in magnetic recording devices, and in memory devices suchas magnetoresistive random access memory (MRAM) and spin torque transfer(STT)-MRAM. An important step in fabricating an array of MTJs is etchtransfer of a pattern in an overlying hard mask through a MTJ stack oflayers to form an array of MTJ nanopillars with a critical dimension(CD) that in state of the art devices is substantially less than 100 nmfrom a top-down view. The etch transfer process typically involves aplurality of etch steps involving reactive ion etch (RIE) and/or ionbeam etch (IBE).

A MTJ stack of layers includes two ferromagnetic layers called the freelayer (FL) and reference layer (RL), and a dielectric layer (tunnelbarrier) between the FL and RL. The RL has a fixed magnetizationpreferably in a perpendicular-to-plane direction (referred to asperpendicular magnetic anisotropy or PMA) while the FL is free to rotateto a direction that is parallel or anti-parallel to the RL magnetizationdirection thereby establishing a “0” or “1” memory state for the MTJ.The magnetoresistive ratio is expressed by dR/R where dR is thedifference in resistance between the two magnetic states when a currentis passed through the MTJ, and R is the minimum resistance value.

The bottommost MTJ layer is usually a non-magnetic seed layer thatpromotes uniform growth in overlying layers, and enhances PMA in theoverlying RL or FL. A capping layer such as Ta is generally formed asthe uppermost MTJ layer and serves as a protective layer duringsubsequent physical and chemical etches. Thus, a single etch transferprocess through the MTJ stack of layers is challenging since there are avariety of materials (magnetic alloys, non-magnetic metals, anddielectric films) that each have a different etch rate when subjected toIBE with Ar or to conventional CH₃OH based RIE. In particular, methanolRIE causes chemical and plasma damage on MTJ sidewalls although there isminimal redeposition of etched material on the sidewalls. On the otherhand, IBE produces no chemical damage and leaves minimal plasma damage,but results in a high degree of redeposited material on MTJ sidewalls.When metal such as Ta is redeposited on the tunnel barrier, shorting mayeasily occur and render the device unusable.

Current technology does not provide a single etch solution fortransferring a hard mask pattern through an entire MTJ stack of layerswithout either a substantial redeposition of one or more MTJ materialson the MTJ sidewalls, or significant damage to the sidewalls. In anycase, removal of material from the sidewalls requires one or more extrasteps that reduce throughput and add cost. Moreover, damaged sidewallsare difficult to repair and often lead to reduced yield and thereforehigher cost per unit of acceptable product. Therefore, a new method foretching a MTJ stack of layers in a single etch process is needed forhigher throughput and lower cost, and the method must maintain orimprove magnetic properties including the magnetoresistive ratio in theMTJ nanopillar. Furthermore, a process flow for etching MTJ sidewalls isdesired that substantially reduces sidewall damage for devices withdiameter (CD) around 60 nm or less.

SUMMARY

One objective of the present disclosure is to provide a method foretching all layers in a MTJ stack below the hard mask with a single etchstep that leaves minimal residue.

A second objective of the present disclosure is to provide a processflow for MTJ etching that satisfies the first objective andsubstantially reduces sidewall damage and associated edge effectscompared with conventional methanol based RIE thereby enabling improveddevice performance, especially for MTJ nanopillars with criticaldimensions ≤60 nm.

According to a preferred embodiment, the first objective is achievedwith a MTJ stack of layers having at least a reference layer, freelayer, a tunnel barrier between the free layer and reference layer, andan uppermost hard mask. In some embodiments, a seed layer is employed asthe bottommost MTJ layer. A pattern comprising a plurality of islandfeatures with the desired critical dimension for the eventual MTJnanopillars is first defined in a photoresist mask layer above the hardmask layer. Preferably, there is a bottom anti-reflective coating (BARC)or a dielectric anti-reflective coating (DARC) between the hard mask andphotoresist mask layer that has better resistance to subsequent etchprocesses than the photoresist mask. The pattern is etch transferredthrough the BARC or DARC by a first RIE or IBE step, and is thentransferred through the hard mask by continuing the first etch step, orby performing a second RIE step comprised of a fluorocarbon orchlorocarbon gas, or by a second IBE step.

According to one embodiment of the present disclosure, the pattern ofisland features is then transferred through the remaining MTJ layers bya RIE step comprising an inert gas, and a chemical such as methanol,ethanol, ammonia, or a combination of CO and NH₃. In particular, Ar ionsor the like provide a physical component to the etch while the one ormore chemicals provide a plasma component for chemical etching. As aresult, inert gas ions or plasma substantially minimize chemical damageto the MTJ sidewalls, and redeposition of etched residue on the MTJsidewalls is significantly reduced by the chemical component. In fact,depending on the composition of the MTJ layers, etch conditions may beoptimized to yield minimal sidewall residue, and substantially lesssidewall damage than in conventional methanol based RIE. Thereafter, anencapsulation layer is deposited on the resulting MTJ nanopillars, andthen a chemical mechanical polish (CMP) process is performed to removeall layers above the hard mask. The CMP process forms a hard mask topsurface that is coplanar with the surrounding encapsulation layer. Froma top-down view, the MTJ nanopillars form an array of circular orelliptical shapes, for example.

In a second embodiment, a process sequence is used to transfer thepattern of island shapes in the hard mask through the remaining MTJlayers thereby generating a plurality of MTJ nanopillars that hasminimal sidewall damage and residue. A first process flow comprises anIBE step and then a separate chemical treatment to convert non-volatileresidue on MTJ sidewalls into a volatile residue. Thereafter, a secondIBE step, plasma sputter etch step, or a thermal treatment is employedto remove the volatile residue. A second process flow includes a RIEstep and then a separate chemical treatment wherein the chemical may beapplied without plasma to transform any sidewall residue into a volatileform. Next, an optional IBE step, plasma sputter etch step, or thermaltreatment is used to remove the volatile residue. In other embodiments,RIE is alternated with IBE before the optional chemical treatment, andoptional volatilization step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a MTJ stack of layers on which aphotoresist pattern has been formed, and shows ions used to transfer thepattern through an underlying BARC or DARC during an etch sequence ofthe present disclosure.

FIG. 2 is a cross-sectional view of a MTJ stack in FIG. 1 after an etchprocess transfers the pattern through the uppermost hard mask MTJ layeraccording to an embodiment of the present disclosure.

FIG. 3 is cross-sectional view of a MTJ nanopillar after an etch thatcombines physical and chemical components is used to transfer the hardmask pattern in FIG. 2 through the remaining MTJ stack of layersaccording to an embodiment described herein.

FIG. 4a is a cross-sectional view of the MTJ stack in FIG. 2 after anion beam etch transfers the hard mask pattern through the MTJ stack oflayers and causes residue to form on MTJ sidewalls.

FIG. 4b is a cross-sectional view of the MTJ stack in FIG. 2 after areactive ion etch transfers the hard mask pattern through the MTJ stackand causes residue to form on MTJ sidewalls.

FIG. 5 is a cross-sectional view of the MTJ nanopillar in FIG. 4a orFIG. 4b after a chemical treatment convents non-volatile residue tovolatile residue on MTJ sidewalls according to an embodiment of thepresent disclosure.

FIG. 6 is a cross-sectional view of the MTJ nanopillar in FIG. 5 after avolatilization step involving IBE, plasma sputter etch, or thermaltreatment is used to remove the volatile residue according to anembodiment of the present disclosure.

FIG. 7 is a cross-sectional view of the MTJ nanopillar in FIG. 6following deposition of an encapsulation layer and planarization toelectrically isolate the MTJ nanopillar from adjacent MTJ nanopillars.

FIG. 8 is a top-down view of a plurality of MTJ nanopillars having acircular shape in an array of rows and columns according to anembodiment of the present disclosure.

FIG. 9 is a flow diagram showing a sequence of steps of forming a MTJnanopillar according to an embodiment of the present disclosure.

FIGS. 10-11 are flow diagrams showing alternative sequences of formingMTJ nanopillars having sidewalls that are essentially free of damage andresidue according to embodiments of the present disclosure.

FIG. 12 is a plot of magnetoresistive ratio vs. MTJ size for MTJs thathave been ion beam etched with no post-cleaning process while FIG. 13 isa similar plot for MTJ nanopillars formed by a combinedphysical/chemical etch followed by an Ar plasma etch volatilization stepaccording to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure is a method of etching a MTJ stack of layerswherein all layers below the hard mask are removed with a single etchprocess comprised of both physical and chemical components to form a MTJnanopillar with sidewalls that have substantially less sidewall damagecompared with conventional methanol based RIE, and minimal residue. Analternative process sequence is provided where the physical and chemicaletchants are alternated, or are in separate steps and followed by achemical treatment and an optional volatilization process to achieveimproved MTJ performance. Although only one MTJ nanopillar is depictedin the drawings with cross-sectional views, one skilled in the art willappreciate that a plurality of MTJ nanopillars is formed in a typicalmemory device pattern. A process is defined as a method that includesone or more steps, and a sequence or process flow according to thepresent disclosure refers to two or more processes in succession.

Referring to FIG. 1, a MTJ stack of layers 1 that will eventually becomea plurality of MTJ nanopillars in a MRAM or STT-MRAM is shown from across-sectional view. The y-axis is perpendicular to the planes of thelayers in the MTJ stack. There is a substrate 10 that in one embodimentis a bottom electrode in a memory device. The bottom electrode may be amultilayer structure and is typically embedded in a dielectric layer(not shown).

MTJ stack 1 is laid down on the substrate 10 and in the exemplaryembodiment has a bottom spin valve configuration wherein a seed layer11, pinned or reference layer 12, tunnel barrier 13, free layer 14, andhard mask 15 are sequentially formed on the substrate. Each of thereference layer and free layer preferably have PMA with a magnetizationaligned in a y-axis direction. In other embodiments, at least oneadditional layer may be included in the aforementioned MTJ stack such asa Hk enhancing layer between the free layer and hard mask that enhancesPMA in the free layer. The seed layer may be comprised of one or more ofNiCr, Ta, Ru, Ti, TaN, Cu, Mg, or other materials typically employed topromote a smooth and uniform grain structure in overlying layers.

The reference layer 12 may have a synthetic anti-parallel (SyAP)configuration represented by AP2/Ru/AP1 where a anti-ferromagneticcoupling layer made of Ru, Rh, or 1 r, for example, is sandwichedbetween an AP2 magnetic layer and an AP1 magnetic layer (not shown). TheAP2 layer, which is also referred to as the outer pinned layer is formedon the seed layer while AP1 is the inner pinned layer and typicallycontacts the tunnel barrier. AP1 and AP2 layers may be comprised ofCoFe, CoFeB, Co, or a combination thereof. In other embodiments, thereference layer may be a laminated stack with inherent PMA such as(Co/Ni)_(n), (CoFe/Ni)_(n), (Co/NiFe)_(n), (Co/Pt)_(n), (Co/Pd)_(n), orthe like where n is the lamination number. Furthermore, a transitionallayer such as CoFeB or Co may be inserted between the uppermost layer inthe laminated stack and the tunnel barrier layer.

The tunnel barrier layer 13 is preferably a metal oxide that is one ofMgO, TiOx, AITiO, MgZnO, Al₂O₃, ZnO, ZrOx, HfOx, or MgTaO. Morepreferably, MgO is selected as the tunnel barrier layer because itprovides the highest magnetoresistive ratio, especially when sandwichedbetween two CoFeB layers, for example.

The free layer 14 may be Co, Fe, CoFe, or an alloy thereof with one orboth of B and Ni, or a multilayer stack comprising a combination of theaforementioned compositions. In another embodiment, the free layer mayhave a non-magnetic moment diluting layer such as Ta or Mg insertedbetween two CoFe or CoFeB layers that are ferromagnetically coupled. Inan alternative embodiment, the free layer has a SyAP configuration suchas FL1/Ru/FL2 where FL1 and FL2 are two magnetic layers that areantiferromagnetically coupled, or is a laminated stack with inherent PMAdescribed previously with respect to the reference layer composition.

The hard mask 15 is also referred to as a capping layer and is typicallycomprised of one or more of Ta, Ru, TaN, Ti, TiN, and W. It should beunderstood that other hard mask materials including MnPt may be selectedin order to provide high etch selectivity relative to underlying MTJlayers during an etch process that forms MTJ nanopillars with sidewallsthat stop on the bottom electrode. All layers in the MTJ stack may bedeposited in a DC sputtering chamber of a sputtering system such as anAnelva C-7100 sputter deposition system that includes ultra high vacuumDC magnetron sputter chambers with multiple targets and at least oneoxidation chamber. Usually, the sputter deposition process involves anargon sputter gas and a base pressure between 5×10⁻⁸ and 5×10⁻⁹ torr.

Once all of the layers 11-15 are laid down, the MTJ stack 1 may beannealed by heating to a temperature between about 360° C. to 400° C.for a period of up to a plurality of hours to grow a bcc structure inthe reference layer, free layer, and tunnel barrier layer therebyenhancing PMA in the reference layer and free layer. The matchingcrystal structure in the aforementioned layers is also believed toimprove the magnetoresistive ratio in the resulting MTJ nanopillars.

As a first step in the MTJ patterning process according to the presentdisclosure, a BARC or DARC layer 16, and a photoresist layer 17 aresequentially coated on the top surface 15 t of the hard mask. BARC orDARC with top surface 16 t has a refractive index that minimizesreflection of light during the subsequent patternwise exposure therebyenabling more uniform island shapes with less CD variation to be formedin the photoresist layer. Next, a conventional patternwise exposure anddeveloper sequence is employed to form a pattern in the photoresistlayer that comprises a plurality of islands with sidewall 20. Asindicated later by a top-down view in FIG. 8, the islands are in anarray with a plurality of rows and columns. However, only one island isshown in FIG. 1 in order to simplify the drawing. Each island has acritical dimension w1 that in some embodiments is between 60 nm and 100nm, and in other embodiments is from about 10 nm to 60 nm thatcorresponds to the CD required in state of the art memory devices. Notethat some devices are circular such that w1 is formed in both of thex-axis and z-axis directions. However, the top-down shape of island 17may be an ellipse or a polygon such that the z-axis dimension isdifferent from the x-axis dimension.

In the initial etch step 30 that may be an IBE with one or more of Ar,Kr, Xe, or Ne, or may comprise RIE with a fluorocarbon or chlorocarbongas, the island shape in photoresist layer 17 is transferred through theBARC or DARC layer 16. Accordingly, sidewall 20 now extends from a topsurface of the photoresist layer to a top surface 15 t of the hard mask15, and CD w1 is duplicated in the DARC or BARC layer. The photoresistlayer may then be removed with a well known method, or is etched awayduring subsequent etch processes.

Referring to FIG. 2, a second etch process 31 is performed to transferthe island shape with sidewall 20 and CD w1 through the hard mask 15. Insome embodiments, a second IBE with inert gas, or a second fluorocarbonor chlorocarbon based RIE may be used. Moreover, the second IBE or RIEmay include oxygen. However, in other embodiments, the presentdisclosure anticipates that the conditions employed for etch process 31are essentially the same as applied in etch process 30 such that theetch transfer through the hard mask may be a continuation of the etchtransfer through DARC or BARC layer 16. In other words, etch 30 in FIG.1 may be continued until stopping on top surface 14 t of the free layer.As mentioned earlier, the etch transfer process through the hard mask islikely to remove any remaining photoresist layer 17 because the etchrate through the latter is generally high relative to the hard mask, andthe hard mask may be substantially thicker than the remainingphotoresist layer once etch process 30 clears the exposed DARC or BARClayer 16. In some embodiments, a passivation step comprised of applyingoxygen plasma or flowing oxygen into the process chamber immediatelyafter the second RIE or IBE is completed, and without breaking a vacuum,is performed to generate a smoother sidewall 20.

In the embodiments described herein, it should be understood that IBEtypically comprises rotating the work piece (wafer) on which the MTJstack of layers is formed. Moreover, the incident or penetration angleof noble gas ions directed at the wafer surface may be between 0° and90°. IBE may be employed in one or more of hard mask etching, MTJetching, cleaning, and volatilization steps described in later sections.On the other hand, RIE is used only for hard mask or MTJ etching,involves a chemical reactant and stationary wafer, and the resultingplasma is limited to a 90° direction or orthogonal to the wafer surface.According to the present disclosure, a plasma sputter etch is employedonly for volatilization or cleaning steps, comprises a noble gas, and isalso limited to a 90° direction (orthogonal to wafer surface).

Referring to FIG. 3, a key feature according to a first embodiment ofthe present disclosure is a single RIE step 32 m that effectivelytransfers the island shape in the hard mask through all of theunderlying MTJ layers 11-14 thereby forming MTJ nanopillar 1 a. Etchstep 32 m comprises a physical component represented by inert gas ionsor plasma, and a chemical component comprising ions or plasma of achemical species that are generated with reactive ion etch conditions.Here, inert gas is defined as a noble gas that is one of Ar, Kr, Ne, andXe. Thus, both of an inert gas and one or more chemicals includingmethanol, ethanol, H₂O₂, H₂O, N₂O, NH₃, and CO are fed into an etchchamber while a plasma is induced with a RF power between 600 Watts and3000 Watts at a temperature proximate to room temperature. It should beunderstood that the RF power applied to a top electrode may be differentthan the RF power applied to a bottom electrode in a RIE chamber.Typically, one or more wafers are held on the bottom electrode duringRIE processing. The resulting ions and plasma in a preferred embodimentare directed orthogonal to the top surface of the substrate along they-axis direction. As a result, sidewall 20 is a continuous surfaceextending from a top surface 15 t of hard mask 15 to top surface 10 t ofthe bottom electrode.

In a preferred embodiment, the sidewall 20 is substantially verticalsuch that CD w1 is substantially maintained in all MTJ layers 11-15.Note that sidewall angle a tends to become more vertical as the methanol(or chemical) content increases in the noble gas/chemical mixture ofstep 32 m. For example, angle a may be proximate to 75° when noble gascontent is 100% but becomes substantially equal to 90° with a chemicalcontent around 50% or greater. Here, the term “content” refers to flowrate ratio. Therefore, a 50:50 flow rate ratio of noble gas:chemicalindicates a 50% chemical content and 50% noble gas content in the RIEgas mixture. Moreover, we have surprisingly found the combined physicaland chemical etching provides for a sidewall that has substantiallyreduced sidewall damage compared with conventional methanol based RIE,and minimal residue.

In the exemplary embodiment, a thickness t of the DARC or BARC layerremains after the etch transfer. However, depending on the initialthickness and composition of layer 16, and the etch conditions, the DARCor BARC layer may be completely removed during etch process 32 m suchthat hard mask top surface 15 t is exposed. Therefore, a hard mask 15 isadvantageously selected that has a high etch rate selectivity to theunderlying MTJ layers so that a substantial thickness of hard maskremains after etch process 32 m.

An optional volatilization step 34 v illustrated in FIG. 6 may beemployed to remove any residue that has accumulated on sidewall 20 atthe end of etch step 32 m. The volatilization preferably comprises anIBE or plasma sputter etch with inert gas wherein Ar+ ions or ions ofKr, He, or Ne that are generated with a RF or DC power are directedorthogonal or with a certain penetration angle toward a top surface 10 tof the substrate. The wafer (not shown) on which the MTJ nanopillar 1 isformed is typically rotated during IBE or is static for a plasma sputteretch during step 34 v. In another embodiment, the volatilization is athermal treatment in an inert or oxidant atmosphere at a temperaturefrom 50° C. to 450° C.

According to a second embodiment shown in FIGS. 4a-4b , FIG. 5, and FIG.6, the present disclosure encompasses a process flow where the chemicalcomponent in the combined etch of the first embodiment is separated fromthe physical etch component. One or two optional steps related to achemical treatment (FIG. 5) and a volatilization (FIG. 6) may beincluded to realize a MTJ nanopillar 1 a with minimal residue and withsubstantially reduced sidewall damage compared with conventionalmethanol based RIE in the prior art.

In FIG. 4a , an IBE 32 i with inert gas is employed to remove MTJ layers11-14 that are not protected by the DARC or BARC layer 16, and by hardmask 15. As a result, MTJ nanopillar 1 a is generated but hassubstantial non-volatile residues 19 on sidewall 20 and on substrate topsurface 10 t surrounding the MTJ nanopillar. Thereafter, in FIG. 5, achemical treatment 33 may be performed to convert the non-volatileresidues to volatile residues 19 x on the sidewall. Although not boundby theory, it is believed the chemical treatment converts metal residuesuch as Ta to an oxide thereof wherein the oxide is more volatile thanthe metal residue. The chemical treatment comprises one or more ofmethanol, ethanol, and ammonia, and a temperature between roomtemperature and 150° C. The chemical treatment is performed in a chamberwithin an Anelva mainframe or the like, or may be conducted in astand-alone process tool outside a mainframe although the latter optionslows throughput. In some cases, the one or more chemicals are injectedinto the chamber with the substrate. Moreover, oxygen may be added tothe chemical treatment chamber to enhance the rate of oxidation of thenon-volatile residue 19. Alternatively, a plasma may be generated fromthe one or more chemicals with RIE conditions in an etch chamber.Preferably, the applied RF power is sufficiently low so that nosignificant damage occurs to MTJ sidewall 20. For example, the RF powermay be maintained between 100 Watts and 800 Watts.

Referring to FIG. 6, volatilization step 34 v may be performed to removethe volatile residue 19 x and comprises an IBE or a plasma sputter etchwith Ar or another inert gas flow, or a thermal treatment in an etchchamber at a temperature between 50° C. and 450° C. for a period up to 5minutes. In either of the IBE, plasma sputter etch, or thermal treatmentembodiments, an oxygen flow may be added to the inert gas flow duringthe volatilization step. Step 34 v may have a preset endpoint time ofduration based on a separate experimental study that establishes avolatilization time for a typical thickness of residue 19 x. When Arplasma sputter etching is selected, RF power is preferably kept at 100Watts or below to avoid damage to MTJ sidewalls. Preferably, the plasmasputter etch is followed immediately in the same process chamber by anencapsulation process described later.

The present disclosure also encompasses an embodiment wherein steps 33and 34 v are performed simultaneously following etch process 32 i. Inparticular, one or more of methanol, ethanol, NH₃, and CO may beintroduced into an etch chamber along with an inert gas flow. A thermaltreatment may be applied at a temperature between 50° C. and 450° C.while the gas mixture is in the etch chamber. In some embodiments, aplasma is generated with a RF power while the gas mixture is in thechamber and with a temperature in the range of 50° C. to 150° C.Alternatively, a plasma sputter etch may be performed at a temperatureproximate to room temperature.

In a third embodiment that represents a modification of the process flowin the second embodiment, a reactive ion etch 32 r shown in FIG. 4b andcomprising one or more chemicals including methanol, ethanol, NH₃, andCO is inserted either before or after step 32 i in FIG. 4a . In thiscase, chemical treatment 33 may not be necessary since step 32 r mayoxidize non-volatile residues 19 generated by a preceding step 32 i toyield volatile residue 19 x on sidewall 20. Volatilization step 34 v maybe employed after steps 32 i and 32 r to remove any volatile residuethat remains on sidewall 20 of MTJ nanopillar 1 a.

In a fourth embodiment, the process flow in the second embodiment ismodified such that etch step 32 i is replaced by etch step 32 r.Chemical treatment step 33 is usually not necessary since step 32 r hasa tendency to serve the same purpose of oxidizing any non-volatileresidue formed on sidewall 20. An optional volatilization step 34 v mayfollow step 32 r to remove any volatile residue formed on sidewall 20 ofMTJ nanopillar 1 a.

Referring to FIG. 7, an encapsulation layer 25 that is comprised of adielectric material is deposited over the MTJ array after cleansidewalls 20 are formed according to one of the preceding embodiments.Preferably, the encapsulation layer has a thickness of 5-250 nm and isone or more of SiN, SiO2, MgO, Al₂O₃, AIN, BN, or the like that isdeposited immediately after volatilization (in-situ) by physical vapordeposition (PVD), chemical vapor deposition (CVD), ion beam deposition(IBD), or atomic layer deposition (ALD) without breaking the vacuum inthe process chamber previously employed for plasma sputter etching instep 34 v. In embodiments where a thermal treatment in a stand-alonetool is employed for step 34 v, then the wafer must be exposed toatmosphere and moved to the encapsulation process chamber.

Thereafter, a chemical mechanical polish (CMP) process is performed toform a top surface 25 t on the encapsulation layer that is coplanar withtop surface 15 t on hard mask 15. In some embodiments, the CMP processremoves any DARC or BARC layer 16 remaining after etch transfer step 32m, 32 i, or 32 r in the previously described embodiments.

Referring to FIG. 8, the plurality of island shapes of the MTJnanopillars formed by an etch process or process flow of the presentdisclosure is depicted from a top-down view after removing overlyinglayers in the memory structure. MTJ nanopillar 1 a is shown in the samerow as MTJ nanopillar 1 b, and MTJ nanopillars 1 c, 1 d are in the samecolumn as MTJ nanopillars 1 a and 1 b, respectively. As explainedpreviously, the MTJ nanopillars are depicted with a circular shape butmay have elliptical shapes in other embodiments. Generally, millions ofMTJ nanopillars are formed in an array but only four are illustratedhere to simplify the drawing.

Thereafter, a top electrode layer comprised of a plurality of parallelconductive lines (not shown) is formed by a conventional method on theMTJ nanopillars and encapsulation layer 25 as appreciated by thoseskilled in the art. A first top electrode line may contact a top surface15 t of MTJ nanopillars 1 a, 1 c while a second top electrode linecontacts top surface 15 t in MTJ nanopillars 1 b, 1 d. Conductive linesin the top electrode layer are preferably formed along the z-axisdirection that is orthogonal to the conductive lines along the x-axisdirection in the bottom electrode layer. Therefore, bottom electrodeline 10 may contact a bottom surface of both MTJ nanopillars 1 a and 1 bwhile a second bottom electrode line 10-1 contacts the bottom surfacesof MTJ nanopillars 1 c and 1 d.

Referring to FIG. 9, a flow diagram is provided for the process flow ofthe first embodiment where a pattern with a CD is formed in the MTJ hardmask layer by an etch process 100 that comprises step 30 or both steps30, 31 described previously. In step 110, a combined physical/chemicaletch 32 m is applied to transfer the pattern through the remaining MTJlayers and thereby form a plurality of MTJ nanopillars. An optionalvolatilization step 114 comprising IBE, plasma etching, or a thermaltreatment is performed after step 110. Finally, an encapsulation layeris formed around the MTJ nanopillars in step 120 to electrically isolatethe MTJ nanopillars from each other.

In FIG. 10, a flow diagram is depicted for the process flow of thesecond embodiment of the present disclosure where step 100 is followedby a physical (IBE) etch 32 i in step 111, a chemical treatment 33 instep 113, volatilization process 34 v in step 114, and thenencapsulation with step 120.

FIG. 11 shows a flow diagram for the process flow of the thirdembodiment of the present disclosure where step 100 is followed by achemical (RIE) etch 32 r in step 112, an IBE step 111, optionalvolatilization step 114, and finally encapsulation in step 120. Inalternative embodiment described previously, IBE step 111 may precedeRIE step 112, followed by optional step 114, and step 120.

We have demonstrated the benefits of the combined physical/chemical etchprocess of the present disclosure with results from an experiment wherea series of MTJ nanopillars with various diameters (w1 in FIG. 8), andreference MTJ nanopillars with the same range of device sizes werefabricated. All process flows described below included etching a DARClayer/Ta hard mask stack with conditions comprising CF4 RIE, 500Watts(top)/52 W (bottom), 50 sccm CF4 only, and 4.5 mT of pressurefollowed by a passivation step comprised of flowing oxygen into theetching chamber. Each MTJ stack of layers in all devices included a MgOtunnel barrier layer between a CoFeB free layer and a CoFeB referencelayer, a Ta hard mask, and a TaN/NiCr seed layer.

Following the etch through the hard mask, reference MTJ nanopillars werefabricated by a conventional method involving an Ar IBE comprised of 450mm IBS (Ion Beam Source) and 800 W of IBS RF power, 200V/950V of G1/G2voltage, 400 mA of G1 current, 60 rpm rotation, 40° and 80° penetrationangles with no subsequent volatilization process. The wafers wereexposed to air between MTJ etching and encapsulation.

According to a process described in the first embodiment, a MTJ stack oflayers 11-14 (FIG. 3) with an overlying patterned hard mask layer 15 wasetched with a RIE step comprising a 50% Ar/50% methanol mixture and a RFpower of 1500 Watts(top)/1100 Watts (bottom) for 60 seconds. The Ar flowrate was 7.5 standard cubic centimeters per minute (sccm) and the CH3OHflow rate was 7.5 sccm. Thereafter, a volatilization step comprised ofAr plasma sputter etching with a RF power of 75 Watts and a 60 sccm flowrate of Ar was employed for a period of 132 seconds at room temperature.

FIG. 12 is a plot of magnetoresistive ratio (DRR) vs. MTJ size (measuredat 125° C.) for the reference MTJ nanopillars that were patterned by theIBE step. MTJ nanopillar size ranges from about 30 nm to 300 nm. Circledregion 60 indicates a significant population of low tails correspondingto devices where redeposited metal residue bridges the MgO tunnelbarrier layer and causes shorts.

In FIG. 13, results are plotted for MTJ nanopillars that were etched bythe 50/50 Ar/CH3OH etch, and then subjected to a volatilization stepwith Ar plasma sputter etch There is clearly a substantial reduction inthe number of devices with low DRR and thus a higher overall DRR,especially for MTJ nanopillar sizes between 30 and 100 nm. DRR is alsomore uniform for each MTJ size in FIG. 13 compared with FIG. 12, whichindicates a more controlled process that is suitable for a manufacturingenvironment. Furthermore, the etch process embodiments disclosed hereinmay be readily implemented in existing manufacturing lines since no newtools or materials are required.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

What is claimed is:
 1. A method comprising: providing a stack ofmagnetic tunnel junction (MTJ) layers on a first electrode wherein thestack of MTJ layers includes a reference layer, a free layer, and atunnel barrier layer between the reference layer and free layer;patterning the stack of MTJ layers by a reactive ion etch that includesa physical component in the form of noble gas ions and a chemicalcomponent; and performing a volatilization step to remove volatileresidue from a sidewall of the patterned stack of MTJ layers after thereactive ion etch, wherein the volatilization step includes an etchingprocess.
 2. The method of claim 1, wherein the noble gas ions areselected from the group consisting of Ar, Kr, Ne, and Xe.
 3. The methodof claim 1, wherein the chemical component includes ethanol, ethanol,H₂O₂, H₂O, N₂O, NH₃, and CO.
 4. The method of claim 1, wherein theetching process includes a process selected from the group consisting ofan ion beam etching process and a plasma sputter etching process.
 5. Themethod of claim 1, wherein the reactive ion etch includes a reactive iongas mixture having a 50:50 flow rate ratio of noble gas:chemicalcomponent.
 6. The method of claim 1, wherein the etching processincludes a process selected from the group consisting of ion beametching and plasma sputter etching.
 7. The method of claim 1, whereinthe etching process further includes applying a flow of oxygen.
 8. Amethod comprising: providing a stack of magnetic tunnel junction (MTJ)layers, wherein the stack of MTJ layers includes a reference layer, afree layer, and a tunnel barrier layer between the reference layer andfree layer; patterning the stack of MTJ layers by performing an etchingprocess that includes an inert gas, wherein a nonvolatile residue isdisposed on sidewalls of the patterned stack of MTJ layers after thepatterning of the stack of MTJ layer; performing a chemical treatment onthe patterned stack of MTJ layers to covert the nonvolatile residue tovolatile residue; and performing a volatilization step to removevolatile residue from the sidewalls of the patterned stack of MTJlayers, wherein the volatilization step includes performing a processselected from the group consisting of ion beam etching, plasma sputteretching and thermal treatment.
 9. The method of claim 8, wherein theetching process is an ion beam etching process.
 10. The method of claim8, wherein the chemical treatment includes an applying a materialselected from the group consisting of methanol, ethanol, ammonia, CO andcombinations thereof.
 11. The method of claim 10, wherein the applyingof the material occurs in the absence of a RF power.
 12. The method ofclaim 8, wherein the selected process for the volatilization step is theion beam etching and includes introducing an oxygen flow.
 13. The methodof claim 8, wherein the selected process for the volatilization step isthe plasma sputter etching and includes applying an inert gas and a RFpower equal to or less than 100 Watts.
 14. The method of claim 8,wherein the selected process for the volatilization step is the thermaltreatment and includes applying a temperature ranging from about 50° C.to about 450° C.
 15. The method of claim 14, wherein the thermaltreatment further includes introducing an oxygen flow.
 16. A methodcomprising: providing a stack of magnetic tunnel junction (MTJ) layers;patterning a layer from the stack of MTJ layers while other layers fromthe stack of MTJ layers are not patterned; patterning the other layersfrom the stack of MTJ layers by performing an etching process thatincludes an inert gas, wherein a metal residue is disposed on sidewallsof the patterned other layers from the stack of MTJ layers after thepatterning of the other layers; and oxidizing the metal residue to forma metal oxide residue on the sidewalls of the patterned other layersfrom the stack of MTJ layers.
 17. The method of claim 16, whereinoxidizing the metal residue to form the metal oxide residue applying achemical selected from the group consisting of methanol, ethanol,ammonia, CO and combinations thereof.
 18. The method of claim 16,further comprising removing the metal oxide from the sidewalls of thepatterned other layers from the stack of MTJ layers.
 19. The method ofclaim 18, wherein the removing of the metal oxide includes performing aprocess selected from the group consisting of ion beam etching, plasmasputter etching and thermal treatment.
 20. The method of claim 16,wherein the patterning of the layer from the stack of MTJ layersincludes performing an etching process selected from the groupconsisting of an ion beam etch with inert gas and a reactive ion etchwith a fluorocarbon or chlorocarbon.